1. Field of the Invention
The present invention relates in general to optical filters and, more specifically to an electrically activated and deactivated liquid crystal infrared filter, suitable for use with a night vision imaging system.
2. Description of the Prior Art
The design of display systems to be used in close proximity with night vision imaging systems (NVIS) is complicated by the extreme sensitivity of NVIS to infrared and near infrared radiation. To avoid overwhelming delicate receiving circuitry with a surfeit of long wavelength light, it is generally desirable to filter any nearby light sources (including electronic displays) to prevent the emission of those radiation components to which the NVIS is most sensitive.
Unfortunately, available infrared filters also block some red, orange and yellow light. As a result, present infrared filters distort the color balance of multicolor electronic displays by attenuating the lower frequency, longer wavelength components of visible light. Furthermore, both color and monochrome displays suffer attenuation when filtered to remove infrared. This is especially true for relatively long wavelength (red, orange or yellow) monochrome displays.
Moreover, for some applications, such as avionics display systems, it is difficult to remove the filter during the daytime and, as a result, color degradation and attenuation is sustained even when night vision protection is not needed.
For those systems where concerns of weight, size and convenience weigh against the mechanical placement and removal of an infrared filter in front of a display, an infrared filter which could be turned on and off electronically would aid the viewer by improving the daytime characteristics of the display. Up until now, there has been no simple device specifically addressed to this problem.
3. Prior Art References
The prior art, typified by the patent to Silverstein et al., U.S. Pat. No. 5,032,007 does not specifically address the problem described above. Silverstein et al., discloses a filter with three layers of controllable color absorptive liquid crystal media, each layer being absorptive to a different wavelength of visible light. By controlling the different layers, a multi-colored display may be created. The disclosed device may also be used to add color highlights to a monochromatic display.
U.S. Pat. No. 4,749,261, to McGlaughlin et al., teaches a polymer encapsulated liquid crystal panel for use in automobile sunroofs. Infrared filtering is but an incidental part of this disclosure. As it is not under user control, it does not address the issue of avoiding filtering out the lower wavelengths of visible light when infrared filtering is not necessary. In the patent granted to Fergason et al., U.S. Pat. No. 5,074,647, a high speed liquid crystal shutter is disclosed. This device is intended for use with welder's helmets and similar devices.
U.S. Pat. No. 3,833,287, granted to Taylor et al., describes the construction and use of guest-host liquid crystal devices which normally absorb unpolarized light. Color guest host liquid crystal devices without polarizers are disclosed in U.S. Pat. No. 4,396,251, granted to Mukoh et al. Although these patents do help form the foundation for the general field of guest host liquid crystal devices, they do nothing to address the specific problem outlined above.
The patent to Suzuki et al., U.S. Pat. No. 4,257,682 teaches a color liquid crystal device utilizing a guest host effect as well as an electrically controlled birefringence effect, to get a two color device. U.S. Pat. No. 4,396,250, granted to Wada et al., discloses multi-layer guest host liquid crystal devices without polarizers. Although both of these disclosures relate to further developments utilizing guest host liquid crystal technology, they do nothing to address the problem of the unwanted side effects of infrared filtering confronted here.
It is, therefore, an object of the present invention to provide a simple and novel electrically activated infrared filter.
It is a further object of this invention to provide an infrared filter for shielding a night vision imaging system from the infrared radiation emitted by a light source, where such shielding may be easily and automatically removed during the daytime.
It is yet another object of the invention to provide a full color electronic display with a filter that can be activated at night to block all display emissions that could adversely affect NVIS equipment.